1. Field
The present disclosure relates generally to wireless communication devices and, more specifically, to methods for frequency acquisition for a forward CDMA channel when the starting frequency is relatively inaccurate.
2. Background
Many wireless networks and mobile stations (wireless handsets) are designed to support one or more of the technical standards based on Code Division Multiple Accessing (CDMA), such as (1) the IS-95 standard promulgated by the Telecommunications Industry Association/Electronic Industry Association, (2) the related 1xEV-DO standard, (3) the UMTS standards offered by a consortium named “3rd Generation Partnership Project” (3GPP) and (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) (the IS-2000 standard). Within such CDMA cellular systems, signals transmitted from a base station to multiple mobile stations are differentiated from one another by unique codes rather than simply by a frequency or time assignment, as in frequency-division multiple accessing (FDMA) or time-division multiple accessing (TDMA). All of the mobile on a single channel in a cellular system use the same wideband carrier frequency. On the forward CDMA channel (also called the downlink channel) from the base station to the mobile station, the base station simultaneously transmits user data to multiple mobile stations in a cell/sector using different codes for transmissions directed to each mobile station. In a direct-sequence CDMA system, the transmitted signals are spread over a frequency band greater than the minimum bandwidth necessary to transmit the data by modulating a carrier signal by the data signal and then modulating the resulting signal again with a wideband spreading signal. Each mobile station acquires the appropriate wideband coded signal for a particular cell/sector through a process of acquiring various code channels that occupy the entire wideband radio channel.
In order to acquire a wideband code channel centered on a predetermined center frequency, the mobile station uses an oscillator to generate a tuning signal to mix-down with, or down-sample, the wideband coded signal. A code channel cannot be successfully acquired if the frequency error is too large relative to the bandwidth of the demodulator. For example, acquiring a code channel with a tuning signal having a relatively large frequency error, such as greater than +/−5 ppm (parts per million), can be difficult. Therefore, CDMA mobile stations typically use voltage-controlled, temperature-compensated crystal oscillators (VCTCXOs) to generate tuning signals having a relatively small frequency error, such as less than +/−2 ppm. Alternatively, a crystal oscillator without the additional circuitry to compensate for the influence of temperature on the output frequency is relatively less expensive to manufacture than a VCTCXO. The frequency error of a crystal oscillator, however, can be relatively large, such as +/−20 ppm. It would be advantageous for a mobile station to perform channel acquisition using such a relatively less expensive crystal oscillator. Thus, a method is sought for acquiring CDMA code channels using a tuning signal having a relatively larger frequency error, such as greater than +/−5 ppm.